Influenza virus is an enveloped, single‑stranded RNA virus of the family Orthomyxoviridae that causes influenza (“flu”) in humans and many animals.
Explanation
The influenza viruses are divided into types A, B, C and D based on internal proteins. Types A and B are responsible for most human disease. Their genomes consist of eight segments of negative‑sense RNA packaged within a lipid envelope studded with two glycoproteins, haemagglutinin (HA) and neuraminidase (NA). Type A viruses circulate in a wide range of mammals and birds and are further classified by their HA and NA subtypes, such as H1N1 or H3N2. Type B viruses lack multiple subtypes but evolve in two lineages (Victoria and Yamagata). The segmented genome allows reassortment when two different strains infect the same cell, giving rise to new combinations of HA and NA.
Influenza viruses spread through respiratory droplets and infect epithelial cells of the upper and lower respiratory tract. The incubation period is usually one to three days. Symptoms include sudden fever, cough, sore throat, muscle aches and malaise; severe cases can progress to viral or bacterial pneumonia, particularly in young children, the elderly and people with chronic illness. The viruses undergo antigenic drift—gradual accumulation of mutations—and antigenic shift—abrupt reassortment events—which enable them to escape existing immunity and cause seasonal epidemics and occasional pandemics. Annual vaccines are updated to match circulating strains and antivirals such as oseltamivir, zanamivir or baloxavir can reduce disease severity when given early.
Notable occurrences
- The 1918 H1N1 influenza A pandemic, often called “Spanish flu,” caused widespread mortality worldwide.
- Influenza A H2N2 (Asian flu) in 1957 and H3N2 (Hong Kong flu) in 1968 were pandemics resulting from antigenic shift.
- The 2009 H1N1 pandemic originated from a swine influenza virus that reassorted with human and avian strains.
- Avian influenza viruses such as H5N1 and H7N9 occasionally infect humans, causing severe disease with high case fatality rates.
- Seasonal influenza B viruses co-circulate with influenza A each winter and contribute substantially to the burden of influenza illness.
Understanding the biology and evolution of influenza viruses underpins surveillance, vaccine design and antiviral development aimed at reducing the impact of this important respiratory pathogen.
Related Terms: antigenic drift, antigenic shift, hemagglutinin, neuraminidase, respiratory virus